Generator excitation system



April 19, 1949; c LYNN 2,467,757

GENERATOR EXCITATION 'SYS'IEM Filed July 2, 1945 WITNESSES: INVENTOR ffl 7 ZJ C/arence A yn/z i, I a 4 ATTORNEY Patented Apr. 19,- 1949 UNITEDSTATES PATENT OFFICE GENERATOR EXCITATION SYSTEM Clarence Lynn, ForestHills, Pa., assignor to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Application July 2, 1945,Serial No. 602,867

12 Claims. 1

The present invention relates to excitation systems foralternating-current generators, and more particularly to excitationsystems of the type in which the direct-current exciter is driven by analternating-current motor supplied from the output of the generator.

Synchronous alternating-current generators require a source ofdirect-current excitation, and this is usually supplied by a separatedirect-current generator, or exciter. The exciter is often directlyconnected to the shaft of the alternating-current generator so as to bedriven by the same prime mover. In many cases, however, it is notdesired to drive the exciter in this manner, and excitation for thealternating-current generator is supplied by a motor-generator setconsisting of an alternating-current motor, energized from the output ofthe generator, which drives the main exciter and, if desired, a pilotexciter for separately exciting the field of the main exciter. Such anexcitation system has certain advantages, but it is subject to a seriousdiiiiculty in case of a drop in voltage of the main generator. Thus, ifa disturbance occurs on the line to which the main generator isconnected which results in a sudden drop in the generator voltage, thespeed of the driving motor of the excitation motor-generator setdecreases, resulting in a drop in the exciter voltage. This, in turn,causes a decrease in the excitation of the main generator, with afurther reduction in its voltage, so that the condition is aggravated.

The principal object of the present invention is to provide anexcitation system for an alternating-current generator, of the typewhich consists of an alternating-current motor driving an exciter, inwhich the field excitation of the exciter is controlled so as tomaintain the exciter voltage unchanged, or to increase the excitervoltage, in case of a decrease in speed of the. driving motor caused bya drop in the voltage.

age of the main exciter unchanged, or to cause across its brushes I2.

it to rise, even though the self-excited component of the exciterexcitation decreases somewhat because of the decrease in speed of thedriving motor.

Other objects and advantages of the invention will be apparent from thefollowing detailed description, taken in connection with theaccompanying drawing, in which:

Figure l is a schematic diagram showing a preferred embodiment of theinvention, and

Figs. 2, 3 and 4 are similar diagrams showing various modifiedembodiments of the invention.

Fig. 1 shows the invention embodied in an excitation system for athree-phase alternatingcurrent generator I driven by a steam turbine 2,

or other suitable prime mover. The generator I is connected to supply athree-phase line 3, and slip rings 4 are mounted on the shaft of thegenerator for efiecting connection to its field winding. The fieldwinding of the generator I is excited by means of a motor-generator setwhich includes an alternating-current motor 5, a main exciter 6, and apilot exciter I. The driving motor 5 is an alternating-current motorwhich may be of any suitable type, such as a squirrel-cage inductionmotor, and the motor 5 is connected to the alternating-current line 3,either directly or through a three-phase transformer 8. The exciter 6,which is driven directly by the motor 5, as indicated, is adirect-current generator, which may be of any suitable construction,having a commutator engaged by brushes 9. The brushes 9 of the exciter 6are connected directly to brushes riding on the slip rings 4 of thegenerator I to supply direct-current excitation to the field winding ofthe generator. The exciter I; has a field winding III which isseparately excited by the pilot exciter I, the field winding III beingconnected directly to the brushes I2 of the pilot exciter.

The pilot exciter I, which is also directly driven by the motor 5, is adirect-current generator of any suitable construction, and has aself-excited shunt field winding II connected A field rheostat I3 may beconnected in series with the self-excited field winding II to adjust thevoltage of the pilot exciter. The pilot exciter I also has aseparately-excited, differential field winding I4 which produces a fluxopposing the flux of the selfexcited field winding II. The differentialfield winding III is energized by a voltage which is proportional to theoutput voltage of the main generator I. This voltage is preferablyobtained by means of a rectifier I5, of any suitable type,

connected across the alternating-current line 3, either directly orthrough a transformer Hi, the differential winding M being connectedacross the direct-current output of the rectifier 15. The rectifier 25has been shown as a singlephase rectifier, but it will be obvious thatif de sired, a three-phase rectifier might be used.

The flux of the separately-excited difierentiaL field winding i4 is madeless than that of" the self-excited field winding H of the pilotexciter,

which may, for example, be accomplished by de--* signing the transformerl 5 and rectifierrl5:so that the output voltage of the rectifier i5 isless than the normal voltage of the pilot excit'en'llacross which theself-excited field H is connected:

Thus, the effect of the difierential field I4 is to reduce the resultantfield excitatiomof th'e' pilot: exciter from what it would be if theself-excited field alone were used. In normal operation, the

field rheostat i3 is adjusted so that the output voltage ofithe pilotexciter Thas the-:propervaluez.

to supply sufficient field excitaticn to the mainvr exciter fi to c'auseits voltageto have the 'prop'er value to excite the field of the maingeneratoill' with the neces's ary exciting currentzt'oi maintain the'generator' voltage atflthei desiredtvalue: If desired; a voltage--regulator of any suitable: type might be used to control the fieldwinding- 'l I of the pilot exciter 7, although: the present system willoperate satisfactorily without a voltage regulator;

In-ca'se of. a drop': in: voltage of the main: gen era'tor I such asmight be? caused by" a disturb.- an ce'on. th1=,- lineii;- thei'speedofithemo'tor 5. will: decrease, thus tending to reduce the: output Voltaages of'both the pilot exciter l'1'andithe"main?=:1

exciter 6". The voltage applied to the difierentialfiel'd winding I 14however; also decrea'se's fproportionally to the-decrease: in voltageof". the

generator I; thus reducing the. differential 'effect' the resultant field excitation of the: pilot 1 exciter tial field is greater thantheslig-ht decrease in plied to the fiel'd winding' of the maingenerator;

I," therefore, remains:- unchanged, or! increases; thus tending to?restore itsvoltagem When the: generator voltage: comes back up to itsnormal: value, the excitation of the difierential. field's winding M isincreased; so. that the resultant field excitation ofethapilotexciterdecreases; re ducing thesexcit'ation of the mainexciter 6.11 tocompensate -for th'e increase in speediof the amt ing imotorw'c a'sthekvoltagevconies up; and system again: reaches:equilibrium-whemthergen:.-:.- erator voltag'ezi'sz restored-v toits normal? value:

It will be=- apparent, therefore; that aniexcita-c-r tio'ni systenrhas-been provided of-i ther. typevin' which the excitation is 'suppliediirom: a motor; generator set driven by "an; alternating-current": motorenergized! from the output of the: main; generator, but i in which adrop'in generaton'volt 1 age results-Jim maintaining. the exciter.voltage? unchanged, orin' increasinglit, so as tozincreaseat:thesgenerator: excitationande thus'tend tomestoret its voltagei The";systen'r thusIa-ctsiz to: maintain: 7 entiallfieldiwindmgs'aiz sothatvthe resultant field i the generator voltage at the desired value,since a change in either direction results in an opposite change inexcitation, so that the voltage is brought back to its normal value.

It will be obvious that various modifications of this system arepossible within the scope of the invention. Thus, as shown in Fig. 2, itis not necessary 'to -havet two separate field windings on the'pilot'exciter; In this-figure; the pilot exciter I is shown as having a singlefield winding 20, which is connected in series with the output of the:rectifier 15 across the brushes of the pilot exciter, the field rheostat2i, if used, also being inzserieswithr,theuwinding 20. The rectifier l5Hisrc'onnected tooppose the pilot exciter voltage, *as indicatedin thefigure, and the effect of this arrangementisobviously exactly equivalentto that of Fig; 1, since the resultant current flowing in the -field'windin is determined by the difference between the pilot excitervoltage and the outputzvoltagei ofwthemectifieral 5:1 The. resultantfieldrexcitation of thewpilotlexciter i, therefore;;

increases upon. :a; deorea'ser-in the.main-generator?-- voltage in?thersame-mannen as described in: cone.

nectionziwith Fig 1 1, and'the: operation Ofitheicsystem to maintainor;inOrease-vthewmain exciter? voltageissthe 521-1118;?

A tur-then modification of-theinvention-is shown; in. Fig. 3;Inethisvfigure; the main-exciter- 6 and: pilotexciter? are drivenbyrthenalternating-cur;-

rent .motor 5 in the: same-mannen as described? above; In this (1358313118 pilot exciter i hasonly'. a self-excited shunts field windingconnected; across-the? pilot exciter brushes: in series with.fieldsrheostata 26,: ThermaimexciteriBahas .a fields; winding 2- which;is: connected in: series with-thee rectifier.- i5-acrossthecoutput'voltage of the pilot" exciter. 1;, the: rectifier?lfixbeingeconnectedt so that; its-ivoltagexopposes thezvoltagerofi thepilot; execiten. Itizwi'll beseenthat inrthisarrangement;

therresultant currenhin the field: winding 21ro'f' the mainrexciten liiwillE be. determined-by thev dif-'=-ferenceabetweenstlieapilotexcitervoltage and the opposingsi:voltagetofiithe: rectifier-5: l5. Thus, if?

thervoltage of the" main generator 1' decreases,-

theioutpnti voltage ofsthe'rectifien I15 willl'decrea's'eproportionally; and the: excitation of the, main;

generator: fiiawill be =increa's'ed;i soas to maintain itssvoltageimspit-e ofith'et drop in speed'of the motor: 5 :resulting; fromithe;decreased generates-.

voltage, .ontoiactually increase the exciter voltage; The operationofs'thisi system; therefore; is sinrilanrtozithateofs the embodimentspreviously: described, except that'itheedifierential voltage de--firivedo from the generator voltage? isapplied di-- rectlyvtowtheimainexciter fieldi instead of to the pilot exciter field:

The:;present;inventionris also applicable to' sys--temssinwwhichanofpilot exciter is;.used;l Thus, in Fig: 4,;the:alternatingecurrent motor 5 drives an"- exciter; which; has. a selfiexcited; shunt; jl, winding 3| and a difierential, separately 'excltedrfieldzwindingflzn The self exciiie'di' winding 3,!issconnectedl-directly acrossthesbnushes of.theex:--

citen-3flriniserieswitha field rheostat" 33. The-4 separately excited,diiierentiail' fields winding 32* is'rconnected' tothez'outpntvoltage.ofthe rectifier?" l 5;.so:-thatxthe:dilferential:excitation-supplied?- by the-winding; 3 2-i'is 'proportionali'toi-the outputv-olt agezofitheageneratorr h. It. willbe apparentithati the;v operation: on: this": embodiment of; the;-inventtibmi'srtheasamet-aszthaisoffEigs 1; since a decreaseav in the:generator voltage will result 2 in: a: come-- spending decreasea'mthe-excitation ofithe: diffen-l excitation of the exciter 30 increasessufficiently to maintain its voltage, or to cause it to increase, inspite of the decrease in speed of the motor 5, thus maintaining theexciting current of the generator l high enough to tend to restore itsvolt-age. It will also be apparent that instead of using two separatefield windings, the rectifier l5 might be connected in series with theself-excited winding 3 l in the manner shown in Fig. 2 for a pilotexciter.

Thus, an excitation system has been provided for alternating-currentgenerators which is ca pable of various embodiments in each of which theexcitation of the main excitor, or of the pilot excitor, includes twocomponents, one of which is a self-excited component, which varies withthe speed of the driving motor, and the other of which is adifierential, separately excited component proportional to the generatorvoltage, the effect being that a drop in generator voltage causes areduction in the differential component and thus a net increase in theresultant excitation, so as to maintain the exciter voltage unchanged,or to increase it, in spite of the drop in speed of the driving motor.

Various embodiments of the invention have been shown and described inorder to illustrate the flexibility of the idea, but it will be apparentthat various other embodiments are also possible, and it is to beunderstood, therefore, that the invention is not limited to theparticular arrangements shown, but in its broadest aspects it includesall equivalent embodiments and modifications which come Within the scopeof the appended claims.

I claim as my invention:

1. An excitation system for an alternatingcurrent generator, saidexcitation system including a direct-current generator for supplyingexciting current to said alternating-current generator, analternating-current induction motor energized from thealternating-current generator for driving said direct-current generator,and means for providing field excitation for the directcurrent generatorin response to a voltage varying with the speed of said driving motorand an opposing voltage proportional to the voltage of thealternating-current generator.

-2. An excitation system for an alternatingcurrent generator, saidexcitation system including a direct-current generator for supplyingexcitin current to said alternating-current generator, analternating-current induction motor energized from thealternating-current generator for driving said direct-current generator,and means for providing field excitation for the directcurrentgenerator, said excitation-providing means including means for providinga voltage varying with the speed of said driving motor, means forproviding a voltage proportional to the voltage of thealternating-current generator, and means for exciting the direct-currentgenerator in response to the difference of said voltages.

3. An excitation system for an alternatingcurrent generator, saidexcitation system including a direct-current generator for supplyingexcitin current to said alternating-current generator, analternating-current induction motor energized from thealternating-current generator for driving said direct-current generator,and means for providing field excitation for" the direct-currentgenerator, said excitation-providing means including means for providinga component of excitation which varies with the speed of said drivingmotor, and means for providing a smaller, opposing component ofexcitation which is proportional to the voltage of the alterroot-currentgenerator, said excitation-providing means includin a self-excited fieldwinding, and a separately-excited difierential field winding, and meansfor exciting said separately-excited field winding from the voltage ofthe alternatingcurrent generator, whereby the resultant excitationderived from said two field windings increases when the voltage of thealternating-current generator decreases.

5. An excitation system for an alternatingcurrent generator, saidexcitation system including a direct-current generator for supplyingexciting current to said alternating-current generator, analternating-current induction motor energized from thealternating-current generator for drivin said direct-current generator,and means for providing field excitation for the direct-currentgenerator, said excitation-providing means including a self-excitedfield winding, and a separately-excited differential field winding, andrectifier means energized from the output of the alternating-currentgenerator for exciting said separately-excited field windingproportionally to the voltage of the alternating-current generator,whereby the resultant excitation derived from said two field windingsincreases when the voltage of the alternating-current generatordecreases.

6. An excitation system for an alternatingcurrent generator, saidexcitation system includ ing a direct-current generator for supplying exciting current to said alternating-current generator, analternating-current induction motor energized from thealternating-current generator for driving said direct-current generator,and means for providing field excitation for the direct-currentgenerator, said excitation-providing means including a field winding andmeans for energizing said field winding in response to the differencebetween a voltage varying with the speed of said driving motor and avoltage proportional to the voltage of the alternating-currentgenerator.

7. An excitation system for an alternatingcurrent generator, saidexcitation system including a direct-current generator for supplyingexcitin current to said alternating-current generator, analternating-current induction motor energized from thealternating-current generator for driving said direct-current generator,and means for providing field excitation for the direct-currentgenerator, said excitation-providing means including a field Winding,means for providin a voltage varying with the speed of said drivingmotor, means for providing a voltage proportional to the voltage of thealternating-current generator, and means for connecting said fieldwinding across said two voltages in series, said voltages being inopposition, whereby the field windin is energized in response to thedifference of the two voltages.

8. An excitation system for an alternating current generator, saidexcitation system including: a: directeeurrent' .generator*forvsupplying; ex, citing current to? said: alternating-current gen--eratonan' alternating-current induction motor energized from thealternating-current genera ton-for driving saiddirect-current generator,and means. for: providing field. excitation for the directpcurrentgenerator; said-excitationeproviding means including: a: field: winding,means for connecting said'field winding'acrossa voltage. whichvaries-with. theu'speedzof saididrivin motor, rectifier-means energizedfrom the output' of. the alternating-current generatomiandimeans forconnecting said. rectifier means. in series with the field-z winding andin; opposition. to the first:-men-- tione'd voltage.

9.: An: excitation, system. for an alternating current generator, saidexcitationsystem including; a direct.-currentv generator: for supplying.ex-- citing current tosaid alternatingreurrent generator;.analternating-current induction motor energized from thealternating-current generator for driving. said direct-currentgenerator, said directcurrentgenerator havingraifield winding, a second:self-excited direct-current generator driven. by

saidxdrivingmotorfor supplying excitation to the.

first mentionedxdirect-current generator, means for" providing-a voltageproportional to the'voltage of." the alternating-current generator, andmeans for connecting. said voltageand the output of the second;direct-current generator in opposition in series with the field;winding. of the first-- mentioned direct current; generator.

10; An excitationr. system for an alternating-- current. generator,said: excitation system including: adirect-current generator forsuppyling eX'-. citing currentto said alternating-current genorator; analternating-current. induction motor energized irom I the alternating.-cur-rent generator. for; driving saiddirect-current generator,saiddimet-current generator having a field Winding, asecondself-excited. direct-current generator driven by said driving-motor. forsupplying excitation. to the first-mentioned direct-current generator,rectifier meansjenergized from the output of the alternating-currentgenerator, and-means for connecting; said rectifier means and-the outputof the second direct-current generator in opposi tionin serieswiththe'field-winding of the first mentioned. directecurrent generator.

1'1 excitation. system for: an". alternatinge current generator, .said1excitationsystem;includ ing a--direct-currentgenerator for supplyingiexciting current to: said. alternating currenti gem orator, analternating-current induction motor; energized fromzthealternating-current generator: for driving said direct-currentgeneraton. said; direct-current generator having a: field winding; asecond direct-current.generator driven-byisaid if alternating-currentmotor anduconnectedito. sup=-- ply exciting current to. the fieldwindingoilthe. first-mentioned directecurrent generator; .v said; seconddirectecurrent: generatorhaving; a self? excited shunt-field windingand. separately:- excited. differential field, Winding, .and. means-forvarying the excitationof said. separatelwexcitedl: fieldwinding'proportionallyto changes in. the: voltage ofthealternating-current-generator:,

12. An excitation: system: for an: alternatlnge current generator, saidexcitation systeminelud-r ing; a direct-current generator for supplying,exciting; current tosaid" alternating eurrenttgenerator; analternating-current. induction motor; energized from thealternating-current gen erator for driving. saida direct-currentgenerator, said;direct-current generator: hav-inga-fieldrwindling, aseconddire'ct-current generator-i driven: by; said alternating-currentmotor. and. conneoted tot supply exciting current to the. fieldwindingiiof. thefirst-mentioned;- direct-cur-rent generatorg; said seconddirect-current generator having a; seli-excited. shunt field-.windinganda separately excited differential field: winding, rectifier-v means:energized from the alternatingcurrent-.generator; andmeans forenergizing said separately-excited field winding irom the output ofvsaid rectifier;

means;

CLARENCE LYNN.

REFERENCES CITED The following references: are-of; recordi inithe fileof this patent":

UNITED STATES; PATENTS Number: Name" Date 1,276,827 TannerAug;.27,.l9i3i 1,874,716: Stoller Aug..30; 1932 1,9i3;952- Powell. June13;,1933,

